Universal Studios attempted to sue New World for spoofing Jaws (1975) , but Steven Spielberg saw the movie in advance and loved it. After that, Universal dropped the lawsuit.
Director Joe Dante once said that he and collaborator/B-movie producer Roger Corman would submit this and other movies to the MPAA, make the suggested cuts to avoid an X-rating, and then reinsert them before the films were distributed to theaters since the MPAA didn't follow up once it had assigned its final rating.
Steven Spielberg described this film as "the best of the Jaws (1975) rip-offs." He and Joe Dante later collaborated on Twilight Zone: The Movie (1983). Richard Dreyfuss, who played Hooper in Jaws, later had a cameo in the opening scene of the 2010 remake (Piranha 3D (2010)).
The piranha were done by attaching rubber puppet fish to sticks.
Heather Menzies-Urich (as Heather Menzies) was concerned that her husband might not approve of a nude scene she was supposed to film, even though she had posed fully nude for Playboy in 1973. So one of the crew members approached a waitress in the restaurant at the Holiday Inn where the director and crew were staying and asked her if she'd like to be in the movie. She was very excited and said yes. He then told her it involved flashing her breasts and she said that was fine with her. So they shot a close-up of her breasts as she lifted her shirt and edited it into the scene to look like Menzies did it.